Photocatalytic Reactions
In chemistry, photocatalysis is the acceleration of a photoreaction in the presence of a catalyst. In catalysed photolysis, light is absorbed by an adsorbed substrate. In photogenerated catalysis, the photocatalytic activity (PCA) depends on the ability of the catalyst to create electron-hole pairs, which generate free radicals (hydroxyl radicals: .OH) able to undergo secondary reactions. Its comprehension has been made possible ever since the discovery of water electrolysis by means of the titanium dioxide (TiO2). Commercial application of the process is called Advanced Oxidation Process (AOP). There are several methods of achieving AOP's, that can but do not necessarily involve TiO2 or even the use of UV light. Generally the defining factor is the production and use of the hydroxyl radical
A principle of photocatalytic reaction is to accelerate the nature's cleaning and purifying process using light as energy. Discovered in 1960's, Dr. Fujishima of Japan found titanium metal, after irradiation by light, could break water molecules into oxygen and hydrogen gas. By restructuring titanium dioxide particles in nano-scale, a number of new physical and chemical properties were discovered. One of these properties was photocatalytic oxidation which accelerated the formation of hydroxyl radical, one of the strongest oxidizing agents created by nature. Using energy found in the UV light, photocatalyst titanium dioxide can breakdown numerous organic substances such as oil grime and hydrocarbons from car exhaust and industrial smog, volatile organic compounds found in various building materials and furniture, organic growth such as fungus and mildew. Titanium dioxide coatings thus may be useful for oxidation. In addition to its photocatalytic oxidation effect, a titanium dioxide coating exhibits a hydrophilic property (or high water-affinity). More specifically, the titanium dioxide coating attracts water moist in the air to form an invisible film of water. This thin film of water allows the substrate to be anti-static so the coated surface may be easily cleaned by rinse of water. Titanium dioxide thus has been incorporated into commodity products such as paint, cosmetics, sun blocks, and etc. Numerous applications have been developed from utilizing photocatalytic reaction.
When photocatalyst titanium dioxide (TiO2) absorbs ultraviolet radiation from sunlight or an illuminated light source (e.g., fluorescent lamps), it produces pairs of electrons and holes. The electron of the valence band of titanium dioxide becomes excited when illuminated by light. The excess energy of this excited electron promotes the electron to the conduction band of titanium dioxide therefore creating the negative-electron (e−) and positive-hole (h+) pair. This stage is referred as the “photo-excitation” state. The energy difference between the valence band and the conduction band is known as the “Band Gap.” Wavelength of the light necessary for photo-excitation is: 1240 (Planck's constant, h)/3.2 ev (band gap energy)=388 nm.
The positive-hole of titanium dioxide breaks apart the water molecule to form hydrogen gas and hydroxyl radical. The negative-electron reacts with oxygen molecule to form super oxide anion. This cycle continues when light is available
Photocatalytic oxidation is achieved when UV light rays are combined with a TiO2 coated filter. This process creates hydroxyl radicals and super-oxide ions, which are highly reactive electrons.
These highly reactive electrons aggressively combine with other elements in the air, such as bacteria and Volatile Organic Compounds (VOCs), harmful pollutants such as formaldehyde, ammonia and many other common contaminants released by building materials and household cleaners generally found in the home. Effective oxidation of the pollutants breaks down into harmless carbon dioxide and water molecules, drastically improving the air quality.
TiO2 as a Photocatalyst
Titanium dioxide (TiO2) is a potent photocatalyst that can break down almost any organic compound when exposed to sunlight. Titanium dioxide is a well-known photocatalyst for water and air treatment as well as for catalytic production of gases. The general scheme for the photocatalytic destruction of organics begins with its excitation by suprabandgap photons, and continues through redox reactions where OH radicals, formed on the photocatalyst surface, play a major role. Titanium dioxide is non-toxic.
TiO2 has been used in the development of a wide range of environmentally beneficial products, including self-cleaning fabrics, auto body finishes, and ceramic tiles. Other experiments with TiO2 involve removing the ripening hormone ethylene from areas where perishable fruits, vegetables, and cut flowers are stored; stripping organic pollutants such as trichloroethylene and methyl-tert-butyl ether from water; and degrading toxins produced by blue-green algae.
Mechanism of Photocatalytic Chemistry of TiO2 Nanotechnology
Biopolymers
With growing environmental concerns over petrochemical products and environmentally harmful practices, new environmentally friendly polymers are being developed as a replacement for petrochemical based plastics. Materials such as PLA (polylactic acid) such as produced by Natureworks (Cargill) are derived from natural and rapidly renewable resources of corn. To date the vast majority of interest and commercialization is the application of PLA has been for disposable packaging and other disposable products. Although thought of as a disposable plastic, PLA has many abilities and functions that can further expand the usage of this environmentally friendly biobased technology.
Polylactic acid is not derived from petrochemical materials, but from the conversion of starch or cellulosic materials such as corn, wheat, sugar cane, and the starch sources into dextrose then into a lactic acid. The lactic acid is then polymerized into a range of polymer products. Because PLA is not petrochemical based, it has functional and processing abilities outside that of petrochemicals.
Plastics, such as acrylic, polystyrene, PE, PP and most all petrochemical plastics, typically block UV. Currently fused quartz mineral is used for UV transparent applications, but is both difficult and expensive to shape or form into shapes. Further, quartz mineral can not be easily softened to fuse nanominerals onto its surface. Currently few materials are UV transparent and most are expensive or classified as a hazardous material. Materials such as quartz or sapphire have been used in some industries providing a high degree of UV stability. These material have limitations in cost and fabrication among others.
Other engineered polymers such as fluropolymers have been used in UV transparent applications, but are hindered by cost and health considerations. Law suits have been won suing company's based on fluropolymer emissions and pollution.
PLA is a thermoplastic polyester derived from field corn of 2-hydroxy lactate (lactic acid) or lactide. The formula of the subunit is: —[O—CH(CH3)-CO]— The alpha-carbon of the monomer is optically active (L-configuration). The polylactic acid-based polymer is typically selected from the group consisting of D-polylactic acid, L-polylactic acid, D,L-polylactic acid, meso-polylactic acid, and any combination of D-polylactic acid, L-polylactic acid, D,L-polylactic acid and meso-polylactic acid. Some, a polylactic acid-based materials include predominantly PLLA (poly-L-Lactic acid). The number average molecular weight may be about 140,000, although a workable range for the polymer is between about 15,000 and about 300,000. In one embodiment, the PLA is L9000™ (Biomer, Germany), apolylactic acid)
Polylactic acid has a relatively high specific gravity as compared to common plastics with a specific gravity closer to engineered plastics such as polycarbonate. Although similar in specific gravity to polycarbonate used in various functional arid optical products, PLA has a much lower refractive index. In addition due to the unique molecular structure and materials, PLA is virtually transparent in UV wavelength spectrum as compared to polycarbonate and common plastics that have very high UV absorption rates. From this PLA does not have visible or UV degradation or yellowing as found in common plastics. UV transparency and a low refractive index can have a myriad of applications.
UV Resistance and UV Transparency
It has been discovered the PLA has very good UV resistance in regards to UV degradation. Various tests have been performed showing that PLA does not yellow when exposed to exterior light. In addition, tests based on UV-visible photospectrometers show that PLA is transparent to UV A, UV B, and in most of the UV C ranges. This shows that the material allows substantially full transmission of UV waves.
Other materials such as polycarbonate have high degrees of clarity in the visible light spectrum but have high degrees of UV absorption. Most polymers are carefully measured for their UV absorption due to the fact that the absorption of UV has a significant relationship to UV degradation of the polymer. Polymers vary greatly in their resistance to weathering. For example, such as polymethylathacrylate (PMMA) and polytetrafluosoethylene (PTFE) are transparent to UV radiation and hence not susceptible to photodegradation. Materials such as PTFE and PMMA are considered “UV Transparent” materials
According to data obtained, the following show a specific wavelength wherein the material starts to absorb UV-visible wavelengths:
PET420 nmPolycarbonate330 nmPLA240 nm
UV or ultra violet radiation is a shorter wavelength than visible light spectra. The following represents the areas of various UV energy classifications:
UV ALong wave (black light)315 to 400 nmUV BUB Medium wave280 to 315 nmUV CShort wave (germicidal)100 to 280
PLA starts absorption at a much shorter UV wavelength and in addition the amount of absorption is lower than that of a high quality PET.
PLA also has a high surface energy. PLA has a similar range of refractive index as fluoropolymers, but with much higher surface energy.
Polylactic acid has a specific gravity typically around the 1.25 range and can produced in a transparent form. Common plastics for optical and other functional applications such as polycarbonate have specific gravities of typically 1.2 to 1.22 but are UV opaque.
Optical properties such as refractive index, UV absorption/transmission and UV resistance are important issues related to optical applications.
Refractive Index
The refractive index or index of refraction is a ratio of the speed of light in a vacuum relative to that speed through a given medium (this quantity does not refer to an angle of refraction, which can be derived from the refractive index using Snell's Law). As light passes from one medium to another, for example from air to water, the result is a bending of light rays at an angle. This physical property occurs because there is a change in the velocity of light going from one medium into another. Refractive index also describes the quantity that light is bent as it passes through a single substance. This involves calculating the angle at which light enters the medium and comparing that with the angle at which the light leaves the medium.
Another view rates each substance with its own refractive index. This is because the velocity of light through the substance is compared as a ratio to the velocity of light in a vacuum. The velocity at which light travels in a vacuum is a physical constant, and the fastest speed at which energy or information can travel. However, light travels slower through any given material, or medium, that is not a vacuum. This is actually a delay from when light enters the material to when it leaves; i.e., when some is absorbed, and another part transmitted. The following shows various refractive indices of plastics:
Specific GravityRefractive IndexPolycarbonate 1.2-1.221.58Polylactic Acid 1.24-1.251.46 note: Range with blending(1.4 to 1.55)
The difference of refractive index between PLA and conventional petrochemical polymers also provides other potential functional features including electrical dielectric strength. The dielectric constant (which is often dependent on wavelength) is the square of the (complex) refractive index in a non-magnetic medium (one with a relative permeability of unity). The refractive index is used for optics in Fresnel equations and Snell's law; while the dielectric constant is used in Maxwell's equations and electronics. The dielectric constant of PLA is lower than conventional petrochemical plastics and has various applications in electrical components and systems.
Fluoropolymers have been investigated for a wide range of optical applications because of their possible optical clarity and because their refractive indices are generally much lower than competing materials such as PMMA and PC. The refractive index for most fluoropolymers is in the region of 1.30 to 1.45 compared with the refractive index for more traditional transparent polymers such as PMMA and PC where it is in the region of 1.5 to 1.6 (or higher). This makes the fluoropolymers suitable for optical technology products such as waveguides, optical filters, fiber gratings and a wide range of optical devices. Specialist ultra-transparent fluoropolymers are also being developed for these applications and for use in rapidly developing CMOS lithography technologies essential for the production of semiconductor devices. The optical clarity and other performance properties of fluoropolymers are opening new markets and opportunities.
The usage of dissimilar materials with various refractive indexes are used for a wide range of applications for antireflective coatings, LCD flat panel screen assemblies, general optical lensing and other similar applications. A lower or different refractive index of PLA in combination with a convention higher refractive index can have unique applications and provide a tool for design of new optical based systems.
Luminous Transmittance
Luminous transmittance for various materials is provided below.
Optical glass99.9PMMA92PC89SAN88PS88ABS79PVC76
Encapsulants for Light Emitting Diodes (LEDs)
Typical encapsulants for LEDs are organic polymeric materials. Encapsulant lifetime is a significant hurdle holding back improved performance of high brightness LEDs (HB LEDs). Conventional LEDs are encapsulated in epoxy resins that, when is use, tend to yellow over time thereby reducing the LED brightness and changing the color rendering index of the light emitted from the light emitting device. This is particularly important for white LEDs. The yellowing of the epoxy is believed to result from decomposition induced by the high operating temperatures of the LED and/or absorption of UV-blue light emitted by the LED.
Another problem that can occur when using conventional epoxy resins is stress-induced breakage of the wire bond on repeated thermal cycling. High brightness LEDs can have heat loads on the order of 100 Watts per square centimeter. Since the coefficients of thermal expansion of epoxy resins typically used as encapsulants are significantly larger than those of the semiconductor layers and the moduli of the epoxies can be high, the embedded wire bond can be stressed to the point of failure on repeated heating and cooling cycles. Thus, there is a need for new photochemically stable and thermally stable encapsulants for LEDs that reduce the stress on the wire bond over many temperature cycles.
With growing research in the area of UV LED, there continues to be a need to improve efficiencies, provide unique UV lensing and optical effects, remove hazardous materials, improve adhesion of encapsulant materials and provide lower cost alternative for LED encapsulant materials.
Currently silicone is a preferred petrochemical polymer used for encapsulating of LED or used within LED lensing due to its refractive index, low UV absorption and other physical and optical properties. Silicone has draw backs with low adhesion properties, petrochemical based, expensive (in optical quality), etc.
Scattering efficiencies is the ratio of photons emitted from the LED lamp to the number of photons emitted from the semiconductor, ship. This accounts for scattering losses in the encapsulant of the lamp.
Encapsulants Based on Silicone
Advantages:
Thermal stability −115 to +260 C
Low Modulus
Low shrinkage
Low moisture absorption
Low ionic content
Low outgassing
Dielectric Strength
Optical clarity—95% transmission at 400 Nm
Refractive index 1.38 to 1.61.
Challenges:
Adhesion to substrate
UV effects on yellowing
Power effect on yellowing.
A key aspect of good high brightness LED packaging design is the physical and optical characteristics of the material used to bond and hold adjacent components together when used as encapsulants, phosphor coatings and lenses. Silicone based materials offer many such advantages.
Silicone Characteristics—Optical:
Optical transmission in the UV-visible region99% @ 400-800 nMHigh clarity95%Refractive index1.38 to 1.58Transmission: Silicones have less than 1% absorption in the UV visible wavelength with very little scattering loss.Note:that certain silicones grades are more prone than others to degradation after prolonged UV exposure which is observed as a characteristic yellowing of the material.
Lighting Lenses
The drop ceiling commercial lighting industry uses metal housings with flourescent tubes to provide indoor lighting. Typically an acrylic or polystyrene sheet lens or diffuser is inserted below the fluorescent tubes to disperse the light in the room and also to protect people if the tubes break.
Currently polystyrene is listed as a probable carcinogen by the US Government as a potential cancer causing agent. Both polystyrene and acrylics are petrochemically derived and are fire accelerators. They have a low limited oxygen index and burn in normal atmospheric oxygen levels. In addition they both have a high smoke index, high heat density and produce toxic smoke during combustion.
Fire safety is of key interest in the interior furnishing area for public occupancy buildings. Current lighting lenses and diffusers are produced from petrochemical based acrylic or polystyrene which has many negative issues related to fire. Acrylics and polystyrene burns vigorously and generates heat rapidly when involved in fire.
Fire safety issues and codes relates to the following issues:
Combustion (limited oxygen index)
Flame Propagation
Ignition Characteristics
Smoke Generation
Heat Generation
Light Diffusing Panels
With growing environmental concerns and demand for more “greener” products, there is an increased demand for materials that are derived from rapidly renewable or recycled resources and replaces hazardous and non-renewable petrochemical products. In addition, there is market demand for a low cost process that can process a wide range of recycled plastics and convert them into highly aesthetic durable goods thus removing this material from landfills.
Certain plastic waste such as compact discs from industrial scrap or postconsumer sources are difficult and expensive to recycle due to the metallization layer of the backside of the CD and the colored ink printing on the front. Other recycled material like polylactic acid biopolymers from the production of water bottles are found in high volumes. In the production of these water bottles, trimming scrap is produced during bottle production and cannot be recycled back into bottles. Other mixed semitransparent plastic waste is also problematic for recycled especially when there is mixed colors with the clear or semitransparent plastic and are virtually impossible to sort.
There is a growing demand in architectural designs to create light transmittance diffusing privacy panels that can allow light to pass, but provide sufficient optical diffusion for privacy. Current technology use transparent sheet plastic with optional inclusions that are melted together to form these panels. This process is slow and expensive. The resulting panels are expensive and aesthetic inclusions are highly labor intensive insofar as they are hand laid up between the sheets prior to pressing processes.
The usage of recycled plastic for simple compression molding into sheets or screw extrusion into sheet is known. Generally, the materials are mixed or blended and fully melted into a homogenous material.
U.S. Pat. Nos. 5,593,625 and 5,635,123 (Riebel) discloses a biocomposite board formed from waste newsprint in combination with a water based soybean resin designed with similar characteristics of a composite wood panel. This composite board requires finishing as hardwoods and is not water proof. This art was designed as a replacement for hardwood based on water based proteinous resin integrated with cellulose.
Filled composite materials are known that are based on thermoplastic polymers and wood or agricultural fiber compounds with various additives, which are manufactured using high volume processes such as injection molding or extrusion. These materials are typically done as homogenous composites designed for structural applications such as decking, windows, fencing, siding and other exterior applications. The art includes thermosetting compounds containing cellulosic fiber as a filler. For example, U.S. Pat. No. 3,367,917 describes a thermosetting melamine resin molding composition containing fibrous filler such as cellulose from about 25% to 42% by weight. U.S. Pat. Nos. 3,407,154 and 3,407,154 describe thermosetting urea-formaldehyde resin molding composition comprising of fusible reactive urea resin and pure cellulosic fibers. This art is based on homogenous composite materials designed for structural applications.
Numerous patents issued in the 1990s concern composite materials comprising polyethylene (high- or low-density, HDPE and LDPE, respectively) and cellulose fibers. U.S. Pat. Nos. 5,082,605, 5,088,910 and 5,096,046 disclose a composite made of 40% to 60% of plastic (LDPE, or a combination of 60% LDPE and 40% HDPE, or having 10-15% of polypropylene of the total amount of plastic) and about 60% to 40% of wood fiber. U.S. Pat. No. 5,474,722 describes a composite material 20% to 80% of which a cellulosic material (ground wood, sawdust, wood flour, rice hulls, etc.) and polyethylene.
U.S. Pat. No. 5,480,602 discloses a composite comprising polypropylene, polyethylene, or their combination along with lignocellulosic particles (50% to 70% by weight) and a polyurethane coupling agent (15 to 3% by weight of the mixture). U.S. Pat. No. 5,516,472 discloses a composite having approximately 26% HDPE and 65% wood flour, extruded in the presence of zinc stearate (2%) as a lubricant along with phenolic resin and polyurethane as minor additives and cross-linking agents (4% and 1.3%, respectively).
U.S. Pat. Nos. 5,827,462, 5,866,264, 6,011,091 and 6,117,924 describe extruded thermoplastic composites comprising 20% to 40% HDPE or polyvinyl chloride, and 50% to 70% of wood flour, along with 0.5%-2% of lubricants (zinc stearate or calcium stearate) and other minor additives. The foregoing four patents contain an example of the composite (Recipe A and B) showing HDPE and PVC at 26% by weight, wood flour at 66%, and the above-indicated amount of lubricants and other minor additives.
U.S. Pat. No. 5,863,480 discloses a thermoplastic composite of polyethylene, polypropylene, vinyls or other extrudable plastics, cellulosic fiber such as saw dust, wood flour, ground rice hulls, etc., fillers and lubricants. The patentees describe the extrusion occurring through a die at a temperature below the melting point of the polymer, so that the deformation of the polymer takes place in the solid phase, making the product biaxially oriented.
Canadian Patent No. 2,278,688 discloses a thermoplastic composite material 50% to 60% of which is polyethylene or polypropylene, 10% to 30% of which is wood powder, and 10% to 35% of which is a silicate (mica).
U.S. Pat. No. 5,952,105 describes a thermoplastic composition comprising sheared poly-coated paper (50% to 70% by weight) and polyethylene (30% to 50%). An example provided in the patent describes making an 80 g batch of a compression molded composite comprising HDPE (39%), a poly-coated paper (scrap milk jugs, 59%) and a coupling agent (Polybond 3009, 2%).
U.S. Pat. No. 5,973,035 by the same authors describes a similar thermoplastic composition comprising sheared paper (50% to 70% by weight) and polyethylene (30% to 50%). An example provided in this patent describes production of an 80 g batch of a compression-molded composite comprising HDPE (39%), sheared scrap newspapers or magazines (59%), and a coupling agent (Fusabond 100D, 2%).
U.S. Pat. No. 6,207,729 describes a similar thermoplastic composition comprising shredded and sheared cellulosic materials (33%-59% by weight) such as old newspapers, magazines, kenaf, kraftboard, etc., HDPE (33% to 50%), calcium carbonate (11% to 17%), and a coupling agent (Fusabond 100D, 2%).
U.S. Pat. No. 6,758,996 teaches that high levels of granulated papermill sludge (up to 70%-75%) can be mixed with synthetic plastics for extrusion or injection molding of a homogenous composite having high strength, high impact resistance, and low flammability for decking products. This art is designed as a homogenous structural material commercially used in extrusion of composite decking.
U.S. Patent Application Publication No. 2005/0241759 discloses a decorative laminate structure having at least two sheets of polycarbonate and at least one decorative image layer there-between two sheets of polycarbonate, and a method of making the decorative laminate structure. Through heat and pressure the sheet layers and the decorative image layer are bonded together resulting in a decorative laminate structure of this invention. This product is intended for use primarily to produce decorative articles which include counter tops, table tops, cabinet doors, game boards, toys, panels for shower stalls, hot tubs, markerboards, indoor and outdoor signs, seamless vanity tops including sink, soap dish, back splash, flooring and others
U.S. Pat. No. 7,615,275 discloses a decorative architectural panel that can be formed using multiple image layers to create one or more three-dimensional effects in a final product. In particular, a reference image layer comprises an extruded sheet having an artistically-designed reference image formed thereon. A next image layer comprises an extruded sheet having a differently sized iteration of the artistically designed reference image. A manufacturer can place the next image layer over the reference image layer, and set the combined image layers such that the final product shows the artistically-designed image in three-dimensions. Additional image layers, such as a color layer, or a layer having embedded objects, can be combined with the stratified product for additional artistic effects. This patent is based on extruded sheet plastics that are laminated together to form a transparent panel with various inclusion pressed in between the transparent sheets. This art teaches that the sheets are fully melted.
Numerous patents have been filed using polylactic acid bioplastics for film and blow molding applications using extrusion processing including U.S. Pat. No. 7,297,394 disclosures using a biodegradable polymer blend for laminating coatings, wraps and other packaging materials. The films are designed for packaging applications wherein the PLA is heated to its crystalline processing temperature and above its melt temperature of 390 degrees F. to obtain optimal clarity.
Decorative solid surfacing materials have been used over the past decade for a myriad of applications including worksurfaces, countertops, architectural components, plaques, tiles, and wall systems all based on various form of hazardous petrochemically derived materials. Materials such as Corian, Avonite and other solid surface materials use forms of acrylic or polyesters in a liquid form with hazardous catalysts wherein decorative chunks of the same material is “floated” within the polymer matrix. These non-environmental materials do not fully have the aesthetics of natural granite due to the fact that particles are uniformly blended within the liquid polymer matrix. The uniformity of floating particles yields a man made and ordered aesthetic appearance.
Other materials have been developed using virgin or blends of recycled clear plastic wherein various inclusion are placed between sheets of these plastics and melted together to form decorative panels. These processes and products are very expensive typically selling for over $20 per square foot and typically are not 100% recycled materials. These materials are limited to long cycles to fully melt and cool the sheet materials into a homogenous sheet form. In addition these layered sheet are homogenous in optical properties.
Generally, the above discussed art is based on the melting of a liquid polymer in forms of virgin, recycled or biobased where the material is mixed and processed above its Tm melting point and to provide a well mixed homogenous material. This art is further based on mixing extrusion processes that requires the melting of the polymer portion to even flow through the machine.
Biolaminates
Biopolymer based biolaminates are environmentally friendly and petrochemical free and also have functional features including UV transparency and high degree of resistance to UV degradation. The highly polar nature of biopolymers provides a high degree of ability to load various levels of fillers or functional materials such as photocatalytic particles or nanoparticle or blends thereof. The UV transparency and resistance of UV degradation provides unique properties for photocatalytics in regards to having photocatalytic in thicker coatings that better accept UV wavelengths based on photocatalytic functionality.
Most current art that teaches about photocatalytics in combination with a binder used as coating state that only very thin layer of coating can be used because of the UV absorption of petrochemical based binders. The same limitation is considered present for petrochemical films wherein the photocatalytic can only be a very thin micro layer on the surface, for example see U.S. Patent Publication No. 2003/0165702.